The rapidly increasing cost of energy emphasizes the need for greater conservation of the energy consumed by both industrial and domestic heating equipment. For example, in home domestic hot water and water space heating equipment, various expedients, such as improving the efficiency of combustion and providing greater insulation to water heating tanks, have been employed to increase the efficiency of the equipment. However, for the water utilized in domestic and industrial hot water systems, a good deal of the energy invested in heating the water is lost when the heated water is discharged to a drain after being used for washing or bathing. In these last mentioned uses, it is not necessary that the heat contained in the water be extracted as it is in space heating; rather, the water is heated primarily because of efficiency in washing. Thus, the energy remaining in the used wash water, which often will be at a temperature substantially above ambient, will be lost in usable form when the waste water is discharged to the sewers.
Some attempt has been made to utilize the heat contained in waste water for other purposes. For example, waste hot water can be simply stored in tanks to allow the water to loose its heat to the surrounding atmosphere. Similarly, the unwanted heat contained in fresh cows milk has been transferred through heat exchange apparatus to provide heat to the milking parlor. However, such systems are not suitable for efficiently recycling the heated water through a domestic hot water system in an economical manner.